LAGOS, Nigeria - Kidnappers freed six Russian sailors and an Estonian who had been abducted from a ship off Nigeria's southern delta, a French oil and gas services company and the military said Thursday.

Paris-based Bourbon SA said in a statement that the sailors were in good health despite facing "difficult conditions while in captivity." The company said the men would leave Nigeria in several days after undergoing medical tests and psychological examinations.

The company statement said Bourbon would not take any questions regarding the sailors' release and did not say if a ransom had been paid to free them. Typically, foreign companies operating in Nigeria's Niger Delta pay cash ransoms to free their employees after negotiating down kidnappers' demands. Foreign hostages can fetch hundreds of thousands of dollars apiece.

Military spokesman Lt. Col. Onyema Nwachukwu said the hostages were freed Wednesday night and were taken under guard to Port Harcourt, the major city in the Niger Delta.

The Russian Foreign Ministry said Thursday that the six Russian sailors freed "feel well and are being provided with everything necessary."

The kidnapping happened during an attack Oct. 15 on a Bourbon ship near the Niger Delta, where the company works closely with oil companies like Chinese-owned Addax Petroleum. Gunmen apparently attacked the Bourbon Liberty 249 and seized the sailors before escaping, the company said.

Another nine sailors onboard the vessel sailed safely away to the company's port in Onne in Nigeria's Rivers state, the company said.

The attack occurred near the Pennington River off Bayelsa state, military officials later said. That's close to the Pennington Export Terminal run by U.S.-based Chevron Corp., which loads crude onto large oil tankers for export to the West.

Foreign oil companies have pumped oil out of the Niger Delta, a region of mangroves and swamps the size of Portugal, for more than 50 years. Despite the billions of dollars flowing into Nigeria's government, many in the delta remain desperately poor, living in polluted waters without access to proper medical care, education or work. The poor conditions sparked an uprising in 2006 by militants and opportunistic criminals who blew up oil pipelines and kidnapped foreign workers.

That violence ebbed in 2009 with a government-sponsored amnesty program that offered ex-fighters monthly payments and job training. However, few in the delta have seen the promised benefits and sporadic kidnappings and attacks continue. The last major kidnapping happened in August, when gunmen attacked a vessel operated by Sea Trucks Group, another oil and gas contractor, and abducted four workers. The workers were later released unharmed.

Bourbon operates support vessels for offshore oil rigs and provides repair, inspection and maintenance services for undersea oil fields, and has a smaller unit that ships commodities like coal, grain and timber worldwide. The company had revenues of €1 billion ($1.3 billion) last year.

The company has been targeted by kidnappers in Nigeria before. In October 2008, pirates near Cameroon's border with Nigeria kidnapped 10 Bourbon workers, all of whom were later released. Pirates also kidnapped and later released nine Bourbon workers seized off Nigeria's coast in January 2009.

In another attack on Bourbon, attackers armed with Kalashnikov assault rifles clashed with Nigerian navy forces in a failed bid to take over an offshore oil platform, then kidnapped three French employees of Bourbon in September 2010. The workers were later released.

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Associated Press writer James Heintz in Moscow contributed to this report.